This invention relates generally to electronic key telephone (EKT) systems. More particularly, this invention relates to a key telephone system using linear delta modulation and TDM techniques to provide a multiplicity of telephone features and functions to remote telephones, while only requiring a two-wire line between each single-line telephone (SLT) and the central equipment and a four-wire line between each key-set telephone (KST) and the central equipment. The key telephone of the present invention is compatible with all types of existing central office (CO) exchange lines.
Telephone system development has progressed from single-line telephones to key-set telephones which allow the user to select, using push-buttons or keys at the key-set telephone, any one of a plurality of telephone lines connected to the key-set telephone. This capability of customer selection of one of a multiplicity of telephone lines has generated considerable user demand for key telephone systems which provide additional features and functions.
In traditional key telephone systems, each telephone line which may be selected by a user at a key-set must be provided to the key-set telephone, along with associated control and power lines. For example, in a key-set telephone which allows the user to select any one of six telephone lines, it has not been uncommon for such a key-set telephone to require a multi-conductor cable having upwards of 25 conductor pairs. Because such multi-conductor cables are costly to purchase and difficult to install and maintain, considerable effort has been directed towards reducing the number of required cable pairs while maintaining at each key-set telephone the available telephone lines as well as additional telephone features and functions. This reduction in required conductors has led to increasingly sophisticated key telephone systems utilizing modern digital techniques and signal processing.
In U.S. Pat. No. 3,843,845 to Ridley, issued Oct. 22, 1974, and assigned to Northern Electric Company Limited, there is disclosed an electronic key telephone system which requires three two-wire pairs between each electronic key telephone and the central control equipment. This disclosed system allows centralized switching at the central control equipment, which greatly reduces the required conductors between each key-set telephone and the central equipment. The three-pair cable between each key-set telephone and the central equipment has analog speech signals on the first pair, switching and indication data on the second pair, and power to the key-set telephone on the third pair.
In an electronic key telephone system recently announced by the Bell Telephone System, a three-pair cable is also required between each key-set telephone and the central control equipment. In this disclosed system, the first and second pairs are used for transmission and reception of various switching and indication data, and the third pair is used for transmission and reception of speech signals in analog form.
In the International Telephone & Telegraph, Inc. electronic key telephone system of the MKS-100 series, a three-pair cable is required between each key-set telephone and the central control equipment. Specifically, in the MKS-100 series, the first and second pairs are used for transmission and reception of various switching and indication data, and the third pair is used for transmission and reception of speech signals in analog form.
In an electronic key telephone system disclosed by the ROLM Corporation in "Computer Sophisitication Yields New Versatility, Economy in Medium-Size DBX," Telephony, Mar. 26, 1975, a 25-pair cable is needed to connect each key-set telephone with the central equipment.
It is therefore apparent that a key telephone system requiring less than a three-pair cable between each key-set telephone and the central equipment would result in substantial cost savings, both for new installations, and for installations utilizing existing building wiring present in traditional single-line telephone systems.